The present invention relates generally to operations associated with wellheads on subterranean wells and, in an embodiment described herein, more particularly provides a wellhead union having a safety interlock.
External access to a wellhead is typically provided by a wellhead union, or "quick union", attached to an upper portion of the wellhead. The union generally consists of a plug sealingly inserted into the interior of a fluid conduit, with the plug maintained in place by a cap threadedly attached to the fluid conduit. In order to gain access to the wellhead, it is normal practice to close a master valve of the wellhead, bleed off any fluid pressure within the wellhead, unthread and remove the cap, and then remove the plug from the fluid conduit.
It will be readily appreciated that, if the fluid pressure in the wellhead is greater than atmospheric pressure at the time the cap is removed, the plug, and possibly the cap, may be violently and uncontrollably propelled from the wellhead. This extremely hazardous condition may easily result in damage of property or loss of life. Even more hazardous conditions are presented when caustic, acidic, or otherwise harmful substances, such as hydrogen sulfide gas, are present in the wellhead at the time the cap is removed.
While the normal practice is to bleed off the fluid pressure within the wellhead before removing the cap, an operator may forget this step, or the operator may attempt to bleed off the pressure but the attempt may fail. One reason that the attempt may fail is that a valve used to bleed off the pressure may become blocked with scale or other debris. Another reason is that a pressure gauge used to indicate fluid pressure within the wellhead may be inaccurate, improperly functioning, or may also be blocked by debris, etc. Thus, an operator, even though he or she may have attempted to bleed off the fluid pressure within the wellhead, still cannot be assured that it has been completely, or partially, bled off.
When the cap is unthreaded from the fluid conduit in ideal conditions, and fluid pressure is present within the wellhead, the cap may be difficult to rotate. This is due to the plug being forced against the cap by the fluid pressure. An operator may recognize that difficulty in rotating the cap may be associated with the presence of fluid pressure, or the operator may equate this difficulty with damaged threads, inadequate leverage, or other possible causes of the difficulty. Additionally, the cap may be unthreaded without difficulty even in the presence of fluid pressure within the wellhead, due to the plug being temporarily stuck in the fluid conduit. Thus, no positive indication of the presence of the fluid pressure within the wellhead is provided.
Although the above discussion has been directed to surface wellheads, it will be readily appreciated that similar problems and hazards exist in association with subsea wellheads. Access to a subsea wellhead is complicated by the fact of its remote location, which also adds to the variety of problems that may be encountered in bleeding off fluid pressure within the wellhead.
From the foregoing, it can be seen that it would be quite desirable to provide a wellhead union which permits convenient access to, fluid communication with, and attachment of other items of equipment to, a wellhead, while preventing such access, fluid communication and attachment when fluid pressure is present within the wellhead. It would also be quite desirable to provide a wellhead union which gives a positive indication of the presence of fluid pressure within the wellhead. It is accordingly an object of the present invention to provide such a wellhead union and methods associated therewith.